They’re still kids and they’re still all right

Ponce de Leon had it all wrong. The fountain of youth isn’t in Florida; it’s in Wisconsin. More specifically, De Leon could have found its preservative powers somewhere between Point Place and Milwaukee.

Need proof? Just look at the characters on “That ‘70s Show” and its predecessor-in-nostalgia “Happy Days.” Through the years, the kids on these shows refuse to age according to the calendar by which the rest of us live.

Back in the real 1970s, Richie, Potsie, Ralph and the Fonz were teens played by twentysomething (and in the Fonz’s case, thirtysomething) actors. Today, in the faux ‘70s of the Fox series that airs tonight at 8, Eric, Kelso, Fez and Donna are teens played by a cast in its mid-20s.

But whereas the “Happy Days” gang eventually graduated from college after six seasons, the development of the “That ‘70s Show” clique seems to be a bit stunted. By what? We won’t hazard a guess.

Suffice it to say that, whatever they’re smoking, the latter group exited high school only last season (the show’s fifth) and now is wrestling with what to do in the next phase of life. For Kelso (Ashton Kutcher), that means becoming a cop, who probably will be as doltish as Officer Barbrady of “South Park.” For Jackie (Mila Kunis) and Hyde (Danny Masterson), it means trying to kiss and make up. And for Eric (Topher Grace) and Donna (Laura Prepon), it means potentially breaking up as Donna prepares to go to school in Madison. As it turns out, Eric won’t be going with her, after his decision to stay home and help Mom take care of ailing Dad.

The saving grace of “That ‘70s Show,” which has its major stars contractually locked up through at least one more season after this one, is that it still manages to be consistently funny despite its fairly standard sitcom trappings and now-implausible casting. Maybe it’s the Wisconsin fountain of youth at work, or maybe it’s just that ‘70s cheese.